Separation of n-propyl acetate from n-propanol by extractive distillation

ABSTRACT

n-Propyl acetate cannot be completely removed from n-propyl acetate - n-propanol - water mixtures by distillation because of the presence of the minimum ternary azeotrope. n-Propyl acetate can be readily removed from mixtures containing it, n-propanol and water by using extractive distillation in which the extractive distillation agent is a higher boiling oxygenated, nitrogenous and/or sulfur containing organic compound or a mixture of these. Examples of effective agents are N,N-dimethylacetamide; acetamide and triethylene glycol; acetamide and N,N-dimethylacetamide and triethanolamine.

This application is a divisional of application No. 624,339 filed June25, 1984.

FIELD OF THE INVENTION

This invention relates to a method for separating n-propyl acetate fromn-propanol using certain higher boiling liquids as the extractive agentin extractive distillation.

DESCRIPTION OF PRIOR ART

Extractive distillation is the method of separating close boilingcompounds or azeotropes by carrying out the distillation in a multiplaterectification column in the presence of an added liquid or liquidmixture, said liquid(s) having a boiling point higher than the compoundsbeing separated. The extractive agent is introduced near the top of thecolumn and flows downward until it reaches the stillpot or reboiler. Itspresence on each plate of the rectification column alters the relativevolatility of the close boiling compounds in a direction to make theseparation on each plate greater and thus require either fewer plates toeffect the same separation or make possible a greater degree ofseparation with the same number of plates. When the compounds to beseparated normally form an azeotrope, the proper agents will cause themto boil separately during extractive distillation and thus make possiblea separation in a rectification column that cannot be done at all whenno agent is present. The extractive agent should boil higher than any ofthe close boiling liquids being separated and not form minimumazeotropes with them. Usually the extractive agent is introduced a fewplates from the top of the column to insure that none of the extractiveagent is carried over with the lowest boiling component. This usuallyrequires that the extractive agent boil twenty Centigrade degrees ormore higher than the lowest boiling component.

At the bottom of a continuous column, the less volatile components ofthe close boiling mixtures and the extractive agent are continuouslyremoved from the column. The usual methods of separation of these twocomponents are the use of another rectification column, cooling andphase separation or solvent extraction.

One of the commercially important ways to manufacture n-propyl acetateis by the catalytic esterification of n-propanol with acetic acid.n-Propyl acetate (b.p.=101.6° C.), n-propanol (b.p.=97.2° C.) and water(b.p.=100° C.) form a ternary azeotrope boiling at 82.2° C. andcontaining 59.5 weight percent n-propyl acetate, 19.5 wt.% n-propanoland 21 wt.% water. n-Propyl acetate also forms a binary azeotrope withn-propanol which boils at 94° C. and contains 37 wt.% n-propyl acetate,and a binary azeotrope with water boiling at 82.4° C. and containing 86wt.% n-propyl acetate. n-Propanol also forms a binary minimum azeotropewith water which boils at 88.1° C. and contains 71.8 wt.% n-propanol.Thus in the esterification of n-propanol with acetic acid to formn-propyl acetate and water, the rectification of this mixture has threebinary and a ternary azeotrope to contend with, and yields the lowestboiling constituent, namely the n-propyl acetate--n-propanol--waterternary azeotrope. It is therefore impossible to produce n-propylacetate from n-propanol and water mixtures by rectification because thelower boiling ternary azeotrope will always come off overhead as theinitial product. Any mixture of n-propyl acetate, n-propanol and watersubjected to rectification at one atmosphere pressure will produce anoverhead product boiling at 82.2° C. and containing 59.5 wt.% n-propylacetate, 19.5 wt.% n-propanol and 21 wt.% water. Extractive distillationwould be an attractive method of effecting the separation of n-propylacetate from n-propanol if agents can be found that (1) will break then-propyl acetate--n-propanol--water azeotrope and (2) are easy torecover from the n-propanol, that is, form no azeotrope with n-propanoland boil sufficiently above n-propanol to make the separation byrectification possible with only a few theoretical plates.

Extractive distillation typically requires the addition of an equalamount to twice as much extractive agent as the n-propylacetate--n-propanol--water on each plate of the rectification column.The extractive agent should be heated to about the same temperature asthe plate into which it is introduced. Thus extractive distillationimposes an additional heat requirement on the column as well as somewhatlarger plates. However this is less than the increase occasioned by theadditional agents required if the separation is done by azeotropicdistillation.

Another consideration in the selection of the extractive distillationagent is its recovery from the bottoms product. The usual method is byrectification in another column. In order to keep the cost of thisoperation to a minimum, an apprciable boiling point difference betweenthe compound being separated and the extractive agent is desirable. Itis also desirable that the extractive agent be miscible with n-propanolotherwise it will form a two-phase azeotrope with the n-propanol in therecovery column and some other method of separation will have to beemployed.

The breaking of this azeotrope by extractive distillation is a newconcept. The closest application of the concept might be the breaking ofthe methyl acetate--methanol described by Berg & Yeh, CHEMICALENGINEERING COMMUNICATIONS, p. 3219-3223, 1984.

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method ofextractive distillation that will enhance the relative volatility ofn-propyl acetate from n-propanol in their separation in a rectificationcolumn. It is a further object of this invention to identify suitableextractive distillation agents which will eliminate the n-propylacetate--n-propanol--water ternary azeotrope and make possible theproduction of pure n-propyl acetate and n-propanol by rectification. Itis a further object of this invention to identify organic compoundswhich, in addition to the above constraints, are stable, can beseparated from n-propanol by rectification with relatively fewtheoretical plates and can be recycled to the extractive distillationcolumn and reused with little decomposition.

SUMMARY OF THE INVENTION

The objects of the invention are provided by a process for separatingn-propyl acetate from n-propanol which entails the use of certainoxygenated, nitrogenous and/or sulfur containing organic compounds asthe agent in extractive distillation.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that certain oxygenated, nitrogenous and/or sulfurcontaining organic compounds, some individually but principally asmixtures, will effectively negate the n-propylacetate--n-propanol--water ternary azeotrope and permit the separationof pure n-propyl acetate from n-propanol by rectification when employedas the agent in extractive distillation. Table 1 lists the compounds,mixtures and approximate proportions that we have found to be effective.The data in Table 1 was obtained in a vapor-liquid equilibrium still. Ineach case, the starting material was the n-propylacetate--n-propanol--water azeotrope. The ratios are the parts by weightof extractive agent used per part of n-propyl acetate--n-propanol--waterazeotrope. The relative volatilities are listed for each of the tworatios employed. The compounds that are effective when used alone areacetamide, dimethylsulfoxide, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, tetraethylene glycol, triethanolamine,N,N-dimethylacetamide and N-methylpyrrolidone. The compounds which areeffective when used in mixtures of two or more components are ethyleneglycol, propylene glycol, 1,3-butanediol, hexylene glycol, diethyleneglycol, triethylene glycol, polyethylene glycol, dipropylene glycol,polypropylene glycol, glycerine, 1,2,6-hexanetriol anddimethylformamide.

                                      TABLE 1                                     __________________________________________________________________________    Extractive Distillation Agents That Are Effective In Separating               n-Propyl acetate From n-Propanol                                                                                 Relative                                   Compounds                   Ratios Volatilities                               __________________________________________________________________________    Dimethylsulfoxide (DMSO)    1      2.23                                       1,4-Butanediol              1.sup.                                                                           6/5.sup.                                                                          1.66                                                                             1.81                                    1,5-Pentanediol             "  "   1.51                                                                             1.81                                    1,6-Hexanediol              "  "   1.28                                                                             1.32                                    Acetamide                   1      2.96                                       Tetraethylene glycol        "  "   1.08                                                                             1.19                                    Triethanolamine             "  "   1.44                                                                             1.54                                    N,N--Dimethylacetamide      "  --  1.55                                       N--Methylpyrrolidone        "  --  1.54                                       DMSO, Ethylene glycol       (1/2).sup.2                                                                      (3/5).sup.2                                                                       2.16                                                                             2.28                                    DMSO, Propylene glycol      "  "   1.62                                                                             1.64                                    DMSO, 1,3-Butanediol        "  "   1.54                                                                             1.60                                    DMSO, 1,5-Pentanediol       "  "   1.79                                                                             1.82                                    DMSO, 1,6-Hexanediol        "  "   1.56                                                                             1.42                                    DMSO, Diethylene glycol     "  "   2.00                                                                             2.10                                    DMSO, Triethylene glycol    "  "   1.45                                                                             1.55                                    DMSO, Tetraethylene glycol  "  "   1.55                                                                             1.63                                    DMSO, Dipropylene glycol    "  "   1.39                                                                             1.41                                    DMSO, Glycerine             "  "   1.25                                                                             1.55                                    DMSO, 1,2,6-Hexanetriol     "  "   1.42                                                                             1.72                                    DMSO, Acetamide             "  "   1.96                                                                             2.00                                    DMSO, Triethanolamine       "  "   1.86                                                                             2.01                                    Dimethylformamide (DMFA), Ethylene glycol                                                                 "  "   1.57                                                                             1.51                                    DMFA, Propylene glycol      "  "   1.43                                                                             1.15                                    DMFA, 1,3-Butanediol        "  "   1.38                                                                             1.36                                    DMFA, 1,4-Butanediol        "  "   1.53                                                                             1.51                                    DMFA, 1,5-Pentanediol       "  "   1.46                                                                             1.38                                    DMFA, 1,6-Hexanediol        "  "   1.09                                                                             1.28                                    DMFA, Diethylene glycol     "  "   1.49                                                                             1.52                                    DMFA, Tetraethylene glycol  "  "   1.22                                                                             1.23                                    DMFA, Dipropylene glycol    "  "   1.24                                                                             1.22                                    DMFA, Glycerine             "  "   1.43                                                                             1.29                                    DMFA, Acetamide             "  "   1.94                                                                             2.04                                    Acetamide, Ethylene glycol  "  "   1.86                                                                             2.08                                    Acetamide, Propylene glycol "  "   1.99                                                                             2.00                                    Acetamide, 1,4-Butanediol   "  "   1.47                                                                             1.43                                    Acetamide, 1,5-Pentanediol  "  "   1.49                                                                             1.75                                    Acetamide, 1,6-Hexanediol   "  "   1.84                                                                             1.90                                    Acetamide, Hexylene glycol  "  "   1.28                                                                             1.50                                    Acetamide, Diethylene glycol                                                                              "  "   1.90                                                                             1.99                                    Acetamide, Triethylene glycol                                                                             "  "   2.07                                                                             2.03                                    Acetamide, Tetraethylene glycol                                                                           "  "   1.58                                                                             1.43                                    DMSO, 1,4-Butanediol        "  "   1.86                                                                             1.64                                    DMSO, N,N--Dimethylacetamide                                                                              "  "   2.06                                                                             2.17                                    DMFA, 1,2,6-Hexanetriol     "  "   1.14                                                                             1.44                                    Acetamide, Polyethylene glycol 300                                                                        "  "   1.54                                                                             1.63                                    Acetamide, Dipropylene glycol                                                                             "  "   1.60                                                                             1.80                                    Acetamide, Polypropylene glycol                                                                           "  "   1.31                                       Acetamide, 1,2,6-Hexanetriol                                                                              "  "   1.53                                                                             1.62                                    Acetamide, Triethanolamine  "  "   1.80                                                                             1.98                                    Acetamide, N,N--Dimethylacetamide,                                                                        "  "   1.83                                                                             1.75                                    N,N--Dimethylacetamide, Ethylene glycol                                                                   "  "   1.66                                                                             1.94                                    N,N--Dimethylacetamide, Propylene glycol                                                                  "  "   1.54                                       N,N--Dimethylacetamide, 1,4-Butanediol                                                                    "  "   1.58                                                                             1.64                                    N,N--Dimethylacetamide, 1,5-Pentanediol                                                                   "  "   2.03                                                                             1.86                                    N,N--Dimethylacetamide, 1,6-Hexanediol                                                                    "  "   1.46                                                                             1.69                                    N,N--Dimethylacetamide, Diethylene glycol                                                                 "  "   1.42                                                                             1.63                                    N,N--Dimethylacetamide, Triethylene glycol                                                                "  "   1.09                                                                             1.51                                    N,N--Dimethylacetamide, Tetraethylene glycol                                                              "  "   1.11                                                                             1.36                                    N,N--Dimethylacetamide, Polyethylene glycol 300                                                           "  "   1.35                                                                             1.62                                    N,N--Dimethylacetamide, Dipropylene glycol                                                                "  "   1.52                                                                             1.48                                    N,N--Dimethylacetamide, Polypropylene glycol                                                              "  "   0.94                                                                             0.91                                    N,N--Dimethylacetamide, 1,2,6-Hexanetriol                                                                 "  "   1.83                                                                             1.74                                    N,N--Dimethylacetamide, Triethanolamine                                                                   "  "   1.71                                                                             1.76                                    N--Methylpyrrolidone, Ethylene glycol                                                                     "  "   1.69                                                                             1.91                                    N--Methylpyrrolidone, 1,6-Hexanediol                                                                      "  "   1.17                                                                             1.18                                    N--Methylpyrrolidone, Tetraethylene glycol                                                                "  "   1.24                                                                             1.16                                    N--Methylpyrrolidone, Triethanolamine                                                                     "  "   1.42                                                                             1.58                                    DMSO, DMFA, Ethylene glycol (1/3).sup.3                                                                      (2/5).sup.3                                                                       2.04                                                                             2.06                                    DMSO, DMFA, 1,3-Butanediol  "  "   1.58                                                                             1.68                                    DMSO, DMFA, 1,4-Butanediol  "  "   1.44                                                                             1.54                                    DMSO, DMFA, 1,5-Pentanediol "  "   1.82                                                                             1.77                                    DMSO, DMFA, 1,6-Hexanediol  "  "   1.62                                                                             1.63                                    DMSO, DMFA, Diethylene glycol                                                                             "  "   1.97                                                                             1.83                                    DMSO, DMFA, Triethylene glycol                                                                            "  "   1.55                                                                             1.53                                    DMSO, DMFA, Tetraethylene glycol                                                                          "  "   1.35                                                                             1.49                                    DMSO, DMFA, Dipropylene glycol                                                                            "  "   1.35                                                                             1.49                                    DMSO, DMFA, Propylene glycol                                                                              "  "   1.58                                                                             1.80                                    DMSO, DMFA, Glycerine       "  "   1.66                                                                             1.56                                    DMSO, DMFA, 1,2,6-Hexanetriol                                                                             "  "   1.50                                                                             1.46                                    DMSO, DMFA, Acetamide       "  "   1.94                                                                             2.16                                    DMSO, DMFA, Triethanolamine "  "   1.70                                                                             2.16                                    DMSO, DMFA, N,N--Dimethylacetamide                                                                        "  "   2.09                                                                             2.12                                    DMSO, N,N--Dimethylacetamide, 1,4-Butanediol                                                              "  "   1.83                                                                             1.78                                    DMSO, N,N--Dimethylacetamide, Polypropylene glycol                                                        "  "   1.64                                                                             1.49                                    DMSO, N-- Methylpyrrolidone, Ethylene glycol                                                              "  "   1.97                                                                             2.09                                    DMSO, N--Methylpyrrolidone, 1,6-Hexanediol                                                                "  "   1.39                                                                             1.13                                    DMSO, N--Methylpyrrolidone, Tetraethylene glycol                                                          "  "   1.56                                                                             1.58                                    DMSO, N--Methylpyrrolidone, Triethanolamine                                                               "  "   1.89                                                                             1.90                                    DMSO, Acetamide, Ethylene glycol                                                                          "  "   2.11                                                                             2.14                                    DMSO, Acetamide, Propylene glycol                                                                         "  "   2.09                                                                             2.06                                    DMSO, Acetamide, 1,4-Butanediol                                                                           "  "   1.84                                                                             1.54                                    DMSO, Acetamide, 1,5-Pentanediol                                                                          "  "   1.69                                                                             1.79                                    DMSO, Acetamide, 1,6-Hexanediol                                                                           "  "   1.75                                                                             1.74                                    DMSO, Acetamide, Diethylene glycol                                                                        "  "   2.34                                                                             2.30                                    DMSO, Acetamide, Triethylene glycol                                                                       "  "   2.00                                                                             2.45                                    DMSO, Acetamide, Polyethylene glycol 300                                                                  "  "   1.64                                                                             1.23                                    DMSO, Acetamide, Dipropylene glycol                                                                       "  "   1.89                                                                             1.83                                    DMSO, Acetamide, Polypropylene glycol                                                                     "  "   1.62                                                                             2.10                                    DMSO, Acetamide, 1,2,6-Hexanetriol                                                                        "  "   1.66                                                                             1.66                                    DMSO, Acetamide, Triethanolamine                                                                          "  "   1.99                                                                             2.13                                    DMSO, Acetamide, N,N--Dimethylacetamide                                                                   "  "   2.26                                                                             2.32                                    DMFA, Acetamide, Ethylene glycol                                                                          "  "   1.71                                                                             1.86                                    DMFA, Acetamide, Propylene glycol                                                                         "  "   1.78                                                                             1.30                                    DMFA, Acetamide, 1,3-Butanediol                                                                           "  "   1.51                                                                             1.55                                    DMFA, Acetamide, 1,5-Pentanediol                                                                          "  "   1.54                                                                             1.31                                    DMFA, Acetamide, Tetraethylene glycol                                                                     "  "   1.42                                                                             1.58                                    DMFA, Acetamide, Dipropylene glycol                                                                       "  "   1.18                                                                             1.39                                    DMFA, Acetamide, Glycerine  "  "   1.54                                                                             1.46                                    DMFA, Acetamide, 1,2,6-Hexanetriol                                                                        "  "   1.41                                                                             1.60                                    DMFA, Acetamide, N,N--Dimethylacetamide                                                                   "  "   1.76                                                                             1.94                                    Acetamide, N,N--Dimethylacetamide, Ethylene glycol                                                        "  "   2.15                                       Acetamide, N,N--Dimethylacetamide, Propylene glycol                                                       "  "   1.48                                                                             1.71                                    Acetamide, N,N--Dimethylacetamide, 1,4-Butanediol                                                         "  "   1.88                                                                             1.90                                    Acetamide, N,N--Dimethylacetamide, 1,5-Pentanediol                                                        "  "   1.38                                                                             1.56                                    Acetamide, N,N--Dimethylacetamide, 1,6-Hexanediol                                                         "  "   2.00                                                                             1.94                                    Acetamide, N,N--Dimethylacetamide, Dipropylene glycol                                                     "  "   1.73                                       Acetamide, N,N--Dimethylacetamide, Polyethylene glycol                                                    "  "   2.39                                                                             1.78                                    Acetamide, N,N--Dimethylacetamide, Triethanolamine                                                        "  "   1.87                                                                             1.86                                    Acetamide, N,N--Dimethylacetamide, Glycerine                                                              "  "   1.43                                                                             1.76                                    Acetamide, N,N--Dimethylacetamide, Tripropylene glycol                                                    "  "   1.27                                                                             1.28                                    N,N--Dimethylacetamide, Glycerine                                                                         (1/2).sup.2                                                                      (3/5).sup.2                                                                       1.25                                                                             1.40                                    N,N--Dimethylacetamide, Tripropylene glycol                                                               "  "   1.08                                                                             1.15                                    __________________________________________________________________________

The two relative volatilities shown in Table 1 correspond to the twodifferent ratios employed. For example, in Table 1, one part of1,4-butanediol with one part of the n-propyl acetate--n-propanol--waterazeotrope gives a relative volatility of 1.66, 6/5 parts of1,4-butanediol give 1.81. One half part of DMSO mixed with one half partof ethylene glycol with one part of the n-propylacetate--n-propanol--water azeotrope gives a relative volatility of2.16, 3/5 parts of DMSO plus 3/5 parts of ethylene glycol gives 2.28.One third parts of DMSO plus 1/3 parts of DMFA plus 1/3 parts ofN,N-dimethylacetamide mixed with one part of the n-propylacetate--n-propanol--water azeotrope gives a relative volatility of2.09; with 2/5 parts, these three give 2.12. In every example in Table1, the starting material is the n-propyl acetate--n-propanol--waterazeotrope which possesses a relative volatility of 1.00.

Several of the compounds and mixtures listed in Table 1 and whoserelative volatility had been determined in the vapor-liquid equilbriumstill, were then evaluated in a glass perforated plate rectificationcolumn possessing 4.5 theoretical plates. The n-propylacetate--n-propanol--water mixture studied contained 59.5 wt.% n-propylacetate, 19.5 wt.% n-propanol and 21 wt.% water which is the ternaryazeotrope composition. In every case the ratio of n-propyl acetate ton-propanol in the overhead is greater than 3.05 and the results aretabulated in Table 2. Without the extractive agent, the overhead wouldbe the azeotrope whose ratio of n-propyl acetate to n-propanol is 3.05.This proves that the extractive agent is negating the azeotrope andmakes the rectification proceed as if the azeotrope no longer existedand brings the more volatile components, n-propyl acetate and water, outas overhead products. It is our belief that this is the first time thatthis has been accomplished for this azeotrope.

The data in Table 2 was obtained in the following manner. The charge was59.5% n-propyl acetate, 19.5% n-propanol and 21% water and after a halfhour of operation in the 4.5 theoretical plate column to establishequilibrium, ethylene glycol at 50° C. and 20 ml/min. was pumped in. Therectification was continued for two hours with sampling of overhead andbottoms after one hour, 1.5 hours and two hours. The average of thethree analyses are shown in Table 2 and was 94.61% n-propyl acetate inthe overhead and 45.52% in the bottoms, both on a water-free basis whichgives a relative volatility of 1.97 of n-propyl acetate to n-propanol.This indicates that the ternary azeotrope has been negated andseparation accomplished. The n-propyl acetate takes off in the form ofits binary azeotrope with water which on condensation, immediately formstwo layers. The solubility of n-propyl acetate in liquid water is only1.6%.

                  TABLE 2                                                         ______________________________________                                        Data From Runs Made In Rectification Column                                                 Wt. % n-Propyl acetate                                                                      Relative                                          Agents          Overhead  Bottoms   Volatility                                ______________________________________                                        Ethylene glycol 94.61     45.52     1.97                                      Propylene glycol                                                                              94.15     67.20     1.58                                      Dimethylformamide (DMFA)                                                                      94.74     63.65     1.68                                      1,4-Butanediol  95.37     56.61     1.81                                      Diethylene glycol                                                                             91.31     44.16     1.78                                      Triethylene glycol                                                                            92.43     54.93     1.67                                      Dipropylene glycol                                                                            91.01     63.85     1.47                                      Tetraethylene glycol                                                                          77.36     44.38     1.38                                      Ethylene glycol (R) +                                                                         94.56     58.50     1.75                                      DMFA (R), 1:1                                                                 Ethylene glycol (R) +                                                                         96.73     47.64     2.17                                      Acetamide 1:1/4                                                               Diethylene glycol (R) +                                                                       94.76     60.06     1.74                                      Acetamide 1:1/4                                                               ______________________________________                                         Notes:                                                                        R indicates agent was reclaimed & reused.                                     Compositions are reported on agent & waterfree basis.                         Agents added at 20 ml/min. & 50° C.                                    Ratios of mixed agents are by weight.                                    

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated byreferring to the data presented in Tables 1 and 2. All of the successfulextractive distillation agents show that n-propyl acetate, n-propanoland water can be separated from their ternary azeotrope by means ofdistillation in a rectification column and that the ease of separationas measured by relative volatility is considerable. Without theseextractive distillation agents, no improvement above with azeotropecomposition will occur in the rectification column. The data also showthat the most attractive agents will operate at a boilup rate low enoughto make this a useful and efficient method of recovering high purityn-propyl acetate from any mixture of these three including the ternaryminimum azeotrope. The stability of the compounds used and the boilingpoint difference is such that complete recovery and recycle isobtainable by a simple distillation and the amount required for make-upis small.

WORKING EXAMPLES Example 1

The n-propyl acetate--n-propanol--water azeotrope is 59.5 wt.% n-propylacetate, 19.5 wt.% n-propanol, 21 wt.% water. Fifty grams of then-propyl acetate--n-propanol--water azeotrope and fifty grams ofN,N-dimethylacetamide were charged to an Othmer type glass vapor-liquidequilibrium still and refluxed for eleven hours. Analysis of the vaporand liquid by gas chromatography gave vapor 75.6% n-propyl acetate,24.4% n-propanol; liquid of 66.7% n-propyl acetate, 33.3% n-propanol.This indicates a relative volatility of 1.55.

Example 2

Fifty grams of the n-propyl acetate--n-propanol--water azeotrope, 25grams of acetamide and 25 grams of triethylene glycol were charged tothe vapor-liquid equilibrium still and refluxed for eleven hours.Analysis indicated a vapor composition of 77.5% n-propyl acetate, 22.5%n-propanol, a liquid composition of 62.4% n-propyl acetate, 37.6%n-propanol which is a relative volatility of 2.07. Five grams ofacetamide and five grams of triethylene glycol were added and refluxingcontinued for another nine hours. Analysis indicated a vapor compositionof 78.6% n-propyl acetate, 21.4% n-propanol, a liquid composition of64.5% n-propyl acetate, 35.5% n-propanol which is a relative volatilityof 2.03.

Example 3

Fifty grams of the n-propyl acetate--n-propanol--water azeotrope, 17grams of acetamide, 17 grams of N,N-dimethylacetamide and 17 grams oftriethanolamine were charged to the vapor-liquid equilibrium still andrefluxed for 15 hours. Analysis indicated a vapor composition of 73.4%n-propyl acetate, 26.6% n-propanol, a liquid composition of 59.7%n-propyl acetate, 40.3% n-propanol which is a relative volatility of1.87. Three grams each of acetamide, N,N-dimethylacetamide andtriethanolamine were added and refluxing continued for another ninehours. Analysis indicated a vapor composition of 73.3% n-propyl acetate,26.7% n-propanol, a liquid composition of 58.4% n-propyl acetate, 41.6n-propanol which is a relative volatility of 1.86.

Example 4

A glass perforated plate rectification column was calibrated withethylbenzene and p-xylene which possesses a relative volatilty of 1.06and found to have 4.5 theoretical plates. A solution comprising 268grams of n-propyl acetate, 88 grams of n-propanol and 94 grams of waterwas placed in the stillpot and heated. When refluxing began, anextractive agent consisting of 80% diethylene glycol and 20% acetamidewas pumped into the column at a rate of 20 ml/min. The temperature ofthe extractive agent as it entered the column was 50° C. Afterestablishing the feed rate of the extractive agent, the heat input tothe n-propyl acetate, n-propanol and water in the stillpot was adjustedto give a total reflux rate of 10-20 ml/min. After one hour ofoperation, the overhead and bottoms samples of approximately two ml.were collected and analysed using gas chromatography. The overheadanalysis was 91.31% n-propyl acetate, 8.69% n-propanol. The bottomsanalysis was 44.16% n-propyl acetate, 55.84% n-propanol. Using thesecompositions in the Fenske equation, with the number of theoreticalplates in the column being 4.5, gave an average relative volatility of1.78 for each theoretical plate. After 11/2 hours of total operatingtime, the overhead and bottoms samples were again taken and analysed.The overhead composition was 90.02% n-propyl acetate, 9.98% n-propanoland the bottoms composition was 45.58% n-propyl acetate, 54.12%n-propanol. This gave an average relative volatility of 1.69 for eachtheoretical plate. After two hours of total operating time, the overheadand bottoms samples were again taken and analysed. The overheadcomposition was 93.43% n-propyl acetate, 6.57% n-propanol and thebottoms composition was 50.66% n-propyl acetate, 49.34% n-propanol. Thisgave an average relative volatility of 1.79 for each theoretical plate.

Example 5

A solution of 268 grams of n-propyl acetate, 88 grams of n-propanol and94 grams of water was placed in the stillpot of the same column used inExample 4 and heat applied. When refluxing began, an extractive agent of80% ethylene glycol and 20% acetamide was fed into the top of the columnat a feed rate of 20 ml/min. and a temperature of 50° C. Afterestablishing the feed rate of the extractive agent, the heat input tothe n-propyl acetate, n-propanol and water in the stillpot was adjustedto give a total reflux rate of 10-20 ml/min. Having established thereflux rate, the column was allowed to operate for one hour. After onehour of steady operation, overhead and bottoms samples of approximatelytwo ml. were collected and analysed using gas chromatography. Theoverhead analysis was 97.03% n-propyl acetate, 2.97% n-propanol, thebottoms analysis was 51.36% n-propyl acetate, 48.64% n-propanol. Usingthese compositions in the Fenske equation with the number of theoreticalplates in the column being 4.5, gave an average relative volatility of2.14 for each theoretical plate. After 11/2 hours of total operation,the overhead composition was 97.28% n-propyl acetate, 2.72% n-propanoland the bottoms composition was 50.63% n-propyl acetate, 49.37%n-propanol. This gave an average relative volatility of 2.20 for eachtheoretical plate. After two hours of total operation, the overheadcomposition was 96.73% n-propyl acetate, 3.27% n-propanol and thebottoms composition was 47.64 n-propyl acetate, 52.36% n-propanol. Thisgave an average relative volatility of 2.17 for each theoretical plate.

The nature of the present invention having been described, what we wishto claim as new and useful and secure by Letters Patent is:
 1. A methodfor recovering n-propyl acetate from a mixture of n-propyl acetate,n-propanol and water which comprises distilling a mixture of n-propylacetate, n-propanol and water in a rectification column in the presenceof about one part of extractive agent per part of n-propylacetate--n-propanol--water mixture, recovering n-propyl acetate andwater as overhead product and obtaining the extractive agent andn-propanol from the stillpot or reboiler, the extractive agent comprisesacetamide.
 2. The method of claim 1 in which the extractive agentcomprises a mixture of acetamide and at least one material from thegroups consisting of ethylene glycol, propylene glycol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, hexylene glycol, diethylene glycol,triethylene glycol, tetraethylene glycol, polyethylene glycol,dipropylene glycol, polypropylene glycol, 1,2,6-hexanetriol,triethanolamine and N,N-dimethylacetamide.
 3. A method for recoveringn-propyl acetate from a mixture of n-propyl acetate, n-propanol andwater which comprises distilling a mixture of n-propyl acetate,n-propanol and water in a rectification column in the presence of aboutone part of extractive agent per part of n-propylacetate--n-propanol--water mixture, recovering n-propyl acetate andwater as overhead product and obtaining the extractive agent andn-propanol from the stillpot or reboiler, the extractive agent comprisesN,N-dimethylacetamide.
 4. The method of claim 3 in which the extractiveagent comprises a mixture of N,N-dimethylacetamide and a least onematerial from the groups consisting of ethylene glycol, propyleneglycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, diethyleneglycol, triethylene glycol, tetraethylene glycol, polyethylene glycol,glycerine, 1,2,6-hexanetriol, and triethanolamine.